Computer-assisted surgery (CAS) systems which employ inertial-based or micro-electro-mechanical sensor (MEMS) trackable members continue to be developed.
One of the principal steps in navigating a bone with inertial sensors is to determine a coordinate system of the bone relative to the sensors, such as to be able to determine the orientation of the bone. With respect to the surgeries involving the leg, such as total knee replacement (TKR) surgery for example, the orientation of the femur and the tibia is determined by their respective mechanical axes. Surgeons commonly use the femoral shaft-tibial shaft angle axes from knee radiographs to estimate the hip-knee-ankle (HKA) angle. HKA angle, which is defined between the mechanical axis of the femur and the mechanical axis of the tibia, can however be measured if the mechanical axis of at least the femur is determined. Evaluations of knee alignment may also serve as a guide for implant management and surgical planning.
When traditional optical CAS navigation systems are used, the determination of the femoral mechanical axis and/or the hip-knee-ankle angle can be achieved, for example, by using two optical bone sensors fixed to the bone or bones at spaced apart locations, each optical sensor having six degrees of freedom (DOF), i.e. three DOF in position and three DOF in orientation. When using trackable members having inertial sensors in an inertial-based CAS system, however, the inertial sensors do not necessarily provide six DOF. While the missing DOF can be calculated if necessary using integrated gyroscope and accelerometer readings, for example, a simpler and more efficient manner to digitize the mechanical axis of a tibia is nonetheless sought.
US Patent Application Publication No. 2012/0053594, the entire content of which is incorporated herein by reference, discloses a device for digitizing a mechanical axis of a tibia using a computer-assisted surgery system. The device includes upper and lower mounting ends interconnected by an alignment rod extending therebetween. At least one trackable member is mounted to the alignment rod of the tool and is in communication with the computer assisted surgery system for providing at least orientation information of the alignment rod. The mechanical axis of the tibia is parallel to the alignment rod and extends between the upper and lower reference points when the tool is mounted on the tibia.
There remains a need for an improved surgical tool and method of using same in conjunction with a CAS system in order to digitize (i.e. digitally acquire) the mechanical axis of at least the femur, and in order to thereby verify the hip-knee-ankle (HKA) angle without requiring a distally extending drop-rod or other physical components clamped to the ankle of the patient.